I believe that is controlled on a per-file basis.
Try editing your template files. That should change it for all new models.
Changing your existing files may require a manual step, or an iLogic script.
Check on the customization forum to see if someone has posted a something to get you started.
Steve Walton
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Me: "This setting needs to be default at the application level because we (like everyone) have hundreds of files."
Reply: "This setting can be applied at the file level."
Me: *bangs head on desk*
The point was missed. This is a design flaw. Why would anyone want some arbitrary obscured orientation to supply the thumbnail?